1. Technical Field of the Present Invention
The present invention generally relates to a polishing apparatus and a method for performing a polishing operation. More specifically, the present invention is an improved method and apparatus for performing polishing using a high viscosity slurry.
2. Background of the Present Invention
In the manufacture of integrated circuits, wafer surface planarity is of extreme importance. Photolithographic processes are typically pushed close to the limit of resolution in order to create maximum circuit density. For a 16 mb dynamic random access memory, minimum critical dimensions, such as wordline and bitline width will initially be in the 0.5.mu.-0.7.mu. range. Since these geometries are photolithographically produced, it is essential that the wafer surface be highly planar so that the electromagnetic radiation used to create a masking layer may be accurately focused at a single level thus resulting in precise imaging over the entire surface of the wafer. If the wafer surface was sufficiently non-planar, the resulting structures would be poorly defined, with the circuit being either non-functional or, at best, endowed with less than optimum performance.
In order to achieve the degree of planarity required to produce ultra high density integrated circuits, Chemical-Mechanical Polishing (CMP) or planarization processes are employed. In general, CMP processes involve holding a semiconductor wafer against a moving polishing surface that is wetted with a chemically reactive/abrasive slurry. Slurries are usually either basic or acidic, and generally contain alumina or silica particles. The polishing surface is typically a planar pad made of relatively soft, porous material such as blown polyurethane. The pad is usually mounted on a planar platten.
Reference now being made to FIG. 1, a diagram is shown illustrating a conventional rotational CMP apparatus 10. The apparatus 10 includes a wafer carrier 11 for holding a semiconductor water 12. A soft, resilient pad 13 is typically placed between the wafer carrier 11 and the wafer 12, and the wafer 12 is generally held against the resilient pad 13 by a partial vacumn, friction, or adhesive, etc. Frictional affixation can be accomplished by placing a resilient backing pad of uniform thickness between the carrier 11 and the wafer 12, the backing pad having a higher co-efficient of friction with respect to the wafer 12 and carrier 11 surface with which it is in contact on opposite sides than the co-efficient of a friction of the wafer 12 with respect to the slurry saturated polishing pad 13. The wafer carrier 11 is designed to be continuously rotated by a drive motor 14. In addition, the wafer carrier 11 is also designed for transverse movement as indicated by the double headed arrow 15. The rotational and transverse movement is intended to reduce variability and material removal rates over the surface of the wafer 12.
The apparatus 10 also includes a rotating platten 16 on which is mounted a polishing pad 17. The platten 16 is relatively large in comparison to the wafer 12 so that during the CMP process, the water can be moved across the surface of the polishing pad 17 by the wafer carrier 11. A low viscosity polishing slurry containing chemically-reactive solution, in which are suspended abrasive particles, is deposited through a supply tube 18 on to the surface of polishing pad 17. The principals of such a conventional CMP apparatus 10 are explained in greater detail in connection with FIG. 2.
Reference now being made to FIG. 2, a diagram is shown illustrating the principles of the conventional rotational CMP process used by the apparatus 10 of FIG. 1. The polishing pad 17 is rotated at an angular velocity of W.sub.p radiance per second (RADS./sec.) about axis 0. The water to be planarized 12 is rotated at an angular velocity of W.sub.W RADS./sec., typically in the same rational sense or rotational sense as the pad. It is easily understood that the linear speed (L) of the polishing pad in centimeters/scc., at any given radius (R) in centimeters from axis will be equal to W.sub.p r. Experience has demonstrated that the rate of removal of material from the wafer surface is related to the speed with which the pad surface makes contact with the wafer surface.
There are a number of disadvantages associated with the conventional CMP process. For example, existing CMP systems employ gravity or other means for forcing the wafer against the polishing surface of the pad with an object that a certain amount of slurry remain disposed between the two structures. In current CMP processes, there is no mechanism for addressing the quantity or quality of slurry disposed between the wafer and the polishing pad. Rather, the hope is that a certain portion of the slurry pumped onto the polishing pad will make it between the wafer and the pad. As a direct result, a large portion of the slurry used during the CMP processions fails to reach the wafer (i.e. it is wasted). Further, the used slurry must have some sort of waste treatment process (e.g. acid/base neutralization, metals removal, organic BOD) reductions, etc.), before it can be disposed or recycled. Other disadvantages of a low viscosity slurry include: undesirable scratchinig of the wafer and lower polishing rates at a high down force.
It would, therefore, be a distinct advantage to have a method and apparatus for CMP processing that would use lower amounts of slurries, and eliminate many of the undesirable characteristics of low viscosity slurries. The present invention provides such a method and apparatus.